DOI: 10.3897/neobiota.105.163728

Authors: Greta J. Frankham, Kristofer M. Helgen, Mark D. B. Eldridge

Abstract: The black rat (Rattus rattus) is one of the most prevalent invasive species globally, having significant impacts across ecosystems as a competitor, predator, and driver of disease for native biota. These impacts have been especially severe on islands. Rat eradication programmes as a conservation management intervention have been undertaken on many islands around the world, including on Australia’s Lord Howe Island (LHI), located in the southwest Pacific, 600 km east of the Australian mainland. In 1918, the black rat was accidentally introduced to LHI, now recognised as a World Heritage-listed biodiverse subtropical island. The impact on LHI’s unique biodiversity was catastrophic, with many endemic taxa becoming extinct or severely declining. A Rodent Eradication Programme (REP) for LHI was launched in early 2019, and by October 2019 detections of rodents had ceased. However, in April 2021, black rats were again detected in the settlement area of LHI. To determine if the rats detected on LHI post-REP were new invaders or remnant individuals, mitochondrial DNA and microsatellite analysis were carried out. Only a single identical CytB haplotype was detected in the pre- and post-REP LHI rats. The black rats sampled from New South Wales (NSW) on the east coast of the Australian mainland and southwest Pacific islands mostly had similar but consistently different CytB haplotypes to LHI rats, or highly divergent haplotypes. Microsatellite data demonstrated that the NSW mainland and LHI pre-REP rat populations were genetically differentiated, with little evidence of gene flow between them in either direction despite potential dispersal routes. The LHI post-REP rats had low genetic diversity, were highly interrelated, and appear to be descended from a small number of individuals. Genetic clustering analyses grouped the LHI post-REP rats with the LHI pre-REP population rather than the NSW mainland population. Thus, both the microsatellite and mtDNA results are consistent with the LHI post-REP rats being the descendants of a small number of the LHI pre-REP population and not new invaders from the NSW mainland population.

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DOI: 10.3897/neobiota.103.145876

Authors: Olivier Gimenez

Abstract: Invasive rodents pose significant ecological, economic, and public health challenges. Robust methods are needed for estimating population abundance to guide effective management. Traditional methods such as capture-recapture are often impractical for invasive species due to ethical, legal and logistical constraints. Here, the application of hierarchical multinomial N-mixture models for estimating the abundance of invasive rodents using removal data is highlighted. Firstly, a simulation study was performed which demonstrated minimal bias, as well as good precision and reliable coverage of confidence intervals across a range of sampling scenarios. Additionally, the consequences of violating the population closure assumption were illustrated by showing how between-occasion dynamics can bias inference. Secondly, removal data was analyzed for two invasive rodent species, namely coypus (Myocastor coypus) in France and muskrats (Ondatra zibethicus) in the Netherlands. Using hierarchical multinomial N-mixture models, the effect of temperature on abundance was examined, while accounting for imperfect and time-varying capture probabilities. Additionally, this study demonstrated how to accommodate spatial variability using random effects, quantify uncertainty in parameter estimates, and account for violations of closure by fitting an open-population model to multi-year data. Taken together, these approaches demonstrate the flexibility and utility of hierarchical models in invasive species management.

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DOI: 10.3897/neobiota.103.159934

Authors: Aysegul Birand, Thomas A. A. Prowse, Phillip Cassey, Paul Q. Thomas

Abstract: Rodent outbreaks have significant ecological, agricultural and economic impacts around the world. Although they can occur with increasing frequency in some parts of the world, current management practices remain ineffective. Gene drives, which are genetic elements with positively biased transmission, have been theoretically shown to eradicate large populations of invasive mice on isolated islands and could potentially serve as pre-emptive tools against mouse plagues. Using a spatially explicit individual-based model, we investigated whether a recently developed murine gene drive, tCRISPR, is effective in suppressing mouse populations within an open agricultural setting that experiences plague-like conditions with varying intensity and duration. We simulated various release strategies involving different temporal and spatial release efforts and measured the reduction in maximum plague population sizes relative to scenarios where no control was applied. Early releases allowed more time for the drive to spread before the onset of plagues, making it the most effective strategy to reduce the impact of plagues. Repeated and spatially extensive release efforts, which were initiated 3 years earlier and continued up to the onset of plagues, resulted in up to 90% reductions in the maximum population sizes compared to plagues without control. Effectiveness declined with later releases; those initiated at the onset of the plagues or later did not result in a decrease in population sizes. We tested whether the total release effort of the most effective strategy can be reduced by incorporating periods without any releases. We showed that pulsed releases with no-release gaps could be similarly effective; however, the timing of both releases was critical. Surprisingly, the lowest release effort, which was the one-time release of a single individual to the release sites, had almost no impact on the first plague, but had a substantial impact on subsequent recurring plagues that occurred 10 years later and reduced the maximum population sizes by ~80%. A newly-developed murine gene drive, tCRISPR, could be an effective pre-emptive control tool for costly mice plagues. Early releases followed by supplementation could eliminate the need to predict or closely monitor their occurrence.

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DOI: 10.3897/neobiota.100.150832

Authors: Claudia Romeo, Elisa Fesce, Lucas A. Wauters, Francesca Santicchia, Peter W. W. Lurz, Andy White, Nicola Ferrari

Abstract: Shared pathogens can alter the interaction between native and alien species resulting in disease-mediated invasions (DMIs). Invasive animals often harbour low-virulence macroparasites, but empirical evidence for macroparasite-driven DMIs is still limited due to their subtile pathological impacts and scarce prominence. Here, we modelled the dynamics of native red squirrels, invasive grey squirrels and their shared nematode Strongyloides robustus to assess whether macroparasites can drive DMIs and lead to native species extinction. Our simulations showed that spill-over of the alien parasite can lead to red squirrel extinction, that grey squirrels can amplify the infection in the native host and that the infection can accelerate the replacement of red squirrels compared to direct competition alone, ultimately facilitating invasion by grey squirrels. These results show that low-virulence macroparasites can potentially mediate animal invasions, suggesting that we are overlooking key drivers of native species decline.

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DOI: 10.3897/neobiota.94.121287

Authors: Paul J. Taillie, Wesley W. Boone IV, Alexandra L. Wilson-Seelig, Robert McCleery

Abstract: Black rats (Rattus rattus) are one of the most widespread invasive animals and have been implicated in the decline of species representing several wildlife taxa, particularly on islands. However, their impact on more closely related species, i.e. rodents, via competition is less well-understood. Using diet similarity as a metric of competition for food resources, we used stable isotopes to compare diets of two populations of black rats to diets of two endangered populations of rice rats (Oryzomys palustris natator and Oryzomys palustris sanibeli) in southern Florida, USA. Specifically, we analysed hair samples from 32 rice rats and 35 black rats for carbon (δ13C) and nitrogen (δ15N) isotopes. In addition, we analysed samples of 129 potential food items to characterise rodent diets using stable isotope mixing models. Despite considerable overlap in isotope-space, we observed differences between rice rats and black rats in the relative composition of plant and animal foods. Specifically, the diets of both populations of rice rats consisted of mostly animal foods, whereas the diets of black rats consisted mostly of plants. In combination with previous work revealing temporal niche partitioning, our results suggest competition between invasive black rats and endangered native rodents may be limited. As such, expensive and logistically complicated efforts to control black rats may have limited success for conserving endangered rodents.

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DOI: 10.3897/neobiota.74.82394

Authors: Aysegul Birand, Phillip Cassey, Joshua V. Ross, Paul Q. Thomas, Thomas A. A. Prowse

Abstract: CRISPR-based gene drives offer novel solutions for controlling invasive alien species, which could ultimately extend eradication efforts to continental scales. Gene drives for suppressing invasive alien vertebrates are now under development. Using a landscape-scale individual-based model, we present the first estimates of times to eradication for long-lived alien mammals. We show that demography and life-history traits interact to determine the scalability of gene drives for vertebrate pest eradication. Notably, optimism around eradicating smaller-bodied pests (rodents and rabbits) with gene-drive technologies does not easily translate into eradication of larger-bodied alien species (cats and foxes).

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DOI: 10.3897/neobiota.63.53811

Authors: Quiterie Duron, Thomas Cornulier, Eric Vidal, Edouard Bourguet, Lise Ruffino

Abstract: On large inhabited islands where complete eradication of alien invasive rodents through the use of poison delivery is often not practical or acceptable, mechanical trapping may represent the only viable option to reduce their impact in areas of high biodiversity value. However, the feasibility of sustained rodent control by trapping remains uncertain under realistic operational constraints. This study aimed to assess the effectiveness of non-toxic rat control strategies through a combination of lethal and live-trapping experiments, and scenario modelling, using the example of a remote montane rainforest of New Caledonia. Rat densities, estimated with spatially-explicit capture-recapture models, fluctuated seasonally (9.5–33.6 ind.ha-1). Capture probability (.01–.25) and home range sizes (HR95, .23–.75 ha) varied greatly according to trapping session, age class, sex and species. Controlling rats through the use of lethal trapping allowed maintaining rat densities at ca. 8 ind.ha-1 over a seven-month period in a 5.5-ha montane forest. Simulation models based on field parameter estimates over a 200-ha pilot management area indicated that without any financial and social constraints, trapping grids with the finest mesh sizes achieved cumulative capture probabilities > .90 after 15 trapping days, but were difficult to implement and sustain with the local workforce. We evaluated the costs and effectiveness of alternative trapping strategies taking into account the prevailing set of local constraints, and identified those that were likely to be successful. Scenario modelling, informed by trapping experiments, is a flexible tool for informing the design of sustainable control programs of island-invasive rodent populations, under idiosyncratic local circumstances.

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DOI: 10.3897/neobiota.58.33118

Authors: Anna Schertler, Wolfgang Rabitsch, Dietmar Moser, Johannes Wessely, Franz Essl

Abstract: The coypu (Myocastor coypus) is a semi-aquatic rodent native to South America which has become invasive in Europe and other parts of the world. Although recently listed as species of European Union concern in the EU Invasive Alien Species Regulation, an analysis of the current European occurrence and of its potential current and future distribution was missing yet. We collected 24,232 coypu records (corresponding to 25,534 grid cells at 5 × 5 km) between 1980 and 2018 from a range of sources and 28 European countries and analysed them spatiotemporally, categorising them into persistence levels. Using logistic regression, we constructed consensus predictions across all persistence levels to depict the potential current distribution of the coypu in Europe and its change under four different climate scenarios for 2041–2060. From all presence grid cells, 45.5% showed at least early signs of establishment (records temporally covering a minimum of one generation length, i.e. 5 years), whereas 9.8% were considered as containing established populations (i.e. three generation lengths of continuous coverage). The mean temperature of the warmest quarter (bio10), mean diurnal temperature range (bio2) and the minimum temperature of the coldest month (bio6) were the most important of the analysed predictors. In total, 42.9% of the study area are classified as suitable under current climatic conditions, of which 72.6% are to current knowledge yet unoccupied; therefore, we show that the coypu has, by far, not yet reached all potentially suitable regions in Europe. Those cover most of temperate Europe (Atlantic, Continental and Pannonian biogeographic region), as well as the coastal regions of the Mediterranean and the Black Sea. A comparison of the suitable and occupied areas showed that none of the affected countries has reached saturation by now. Under climate change scenarios, suitable areas will slightly shift towards Northern regions, while a general decrease in suitability is predicted for Southern and Central Europe (overall decrease of suitable areas 2–8% depending on the scenario). Nevertheless, most regions that are currently suitable for coypus are likely to be so in the future. We highlight the need to further investigate upper temperature limits in order to properly interpret future climatic suitability for the coypu in Southern Europe. Based on our results, we identify regions that are most at risk for future invasions and provide management recommendations. We hope that this study will help to improve the allocation of efforts for future coypu research and contribute to harmonised management, which is essential to reduce negative impacts of the coypu and to prevent further spread in Europe.

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DOI: 10.3897/neobiota.56.49581

Authors: Ryan M. Utz, Alysha Slater, Hannah R. Rosche, Walter P. Carson

Abstract: Monospecific stands of invasive plants can dramatically restructure habitat for fauna, thereby elevating population densities or promoting foraging of consumer species who benefit in the altered habitat. For example, dense stands of invasive plants may protect small mammals from predators, which in turn could increase foraging pressure on seeds that small mammals feed upon. We used a before-after, control-impact experimental design to test whether small mammal capture rates were higher and giving-up densities (GUDs) lower beneath dense stands of Berberis thunbergii, an invasive shrub with a rapidly expanding range throughout eastern North America. Our experimental design included three plot categories: 1) plots heavily invaded by B. thunbergii, 2) control plots lacking invasive shrub cover, and 3) invaded plots where we eradicated B. thunbergii midway through the study. Although our overall small mammal capture rate was low, small mammal captures were 65% higher in B. thunbergii invaded habitat relative to control plots and eradication lowered capture rates by 77%. GUDs were also 26% higher within B. thunbergii relative to control plots and eradication decreased GUDs by 65%. Our findings suggest that small mammals perceive dense stands of B. thunbergii as relatively safe foraging habitat. Prior surveys within our study locations revealed dramatically depressed tree seedling densities under B. thunbergii, thus invasive plants may promote intensive foraging by small mammals and reduce recruitment for species with foraged seeds or seedlings.

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DOI: 10.3897/neobiota.32.8884

Authors: Catherine S. Jarnevich, Nicholas E. Young, Trevor R. Sheffels, Jacoby Carter, Mark D. Sytsma, Colin Talbert

Abstract: Invasive species provide a unique opportunity to evaluate factors controlling biogeographic distributions; we can consider introduction success as an experiment testing suitability of environmental conditions. Predicting potential distributions of spreading species is not easy, and forecasting potential distributions with changing climate is even more difficult. Using the globally invasive coypu (Myocastor coypus [Molina, 1782]), we evaluate and compare the utility of a simplistic ecophysiological based model and a correlative model to predict current and future distribution. The ecophysiological model was based on winter temperature relationships with nutria survival. We developed correlative statistical models using the Software for Assisted Habitat Modeling and biologically relevant climate data with a global extent. We applied the ecophysiological based model to several global circulation model (GCM) predictions for mid-century. We used global coypu introduction data to evaluate these models and to explore a hypothesized physiological limitation, finding general agreement with known coypu distribution locally and globally and support for an upper thermal tolerance threshold. Global circulation model based model results showed variability in coypu predicted distribution among GCMs, but had general agreement of increasing suitable area in the USA. Our methods highlighted the dynamic nature of the edges of the coypu distribution due to climate non-equilibrium, and uncertainty associated with forecasting future distributions. Areas deemed suitable habitat, especially those on the edge of the current known range, could be used for early detection of the spread of coypu populations for management purposes. Combining approaches can be beneficial to predicting potential distributions of invasive species now and in the future and in exploring hypotheses of factors controlling distributions.

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DOI: 10.3897/neobiota.24.8433

Authors: Michael A. Tabak, Sally Poncet, Ken Passfield, Carlos Martinez del Rio

Abstract: Non-native rats (Rattus spp.) threaten native island species worldwide. Efforts to eradicate them from islands have increased in frequency and become more ambitious in recent years. However, the long-term success of some eradication efforts has been compromised by the ability of rats, particularly Norway rats (Rattus norvegicus) which are good swimmers, to recolonize islands following eradications. In the Falkland Islands, an archipelago in the South Atlantic Ocean, the distance of 250 m between islands (once suggested as the minimum separation distance for an effective barrier to recolonization) has shown to be insufficient. Norway rats are present on about half of the 503 islands in the Falklands. Bird diversity is lower on islands with rats and two vulnerable passerine species, Troglodytes cobbi (the only endemic Falkland Islands passerine) and Cinclodes antarcticus, have greatly reduced abundances and/or are absent on islands with rats. We used logistic regression models to investigate the potential factors that may determine the presence of Norway rats on 158 islands in the Falkland Islands. Our models included island area, distance to the nearest rat-infested island, island location, and the history of island use by humans as driving variables. Models best supported by data included only distance to the nearest potential source of rats and island area, but the relative magnitude of the effect of distance and area on the presence of rats varied depending on whether islands were in the eastern or western sector of the archipelago. The human use of an island was not a significant parameter in any models. A very large fraction (72%) of islands within 500 m of the nearest potential rat source had rats, but 97% of islands farther than 1,000 m away from potential rat sources were free of rats.

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DOI: 10.3897/neobiota.11.1819

Authors: Arijana Barun, Daniel Simberloff, Nikola Tvrtkovic, Michel Pascal

Abstract: The small Indian mongoose (Herpestes auropunctatus) is one of the world’s 100 worst invasive species (IUCN 2000). It has negative impacts on several small mammals on islands where it was introduced. We assess the abundance of small mammal populations and the activity time of introduced ship rats (Rattus rattus) on three mongoose-infested and three mongoose-free islands in the Adriatic Sea, Croatia. We set up three transects on each island with a trapping system consisting of 30 small live traps to capture small mammals under 30 grams and 30 larger traps to capture ship rats and mongooses, on each transect. Our results support an already large but mostly speculative literature that suggests inability of the small Indian mongoose to reduce high abundances of introduced R. rattus. Further, we suggest that the low abundance of native small mammals is probably not solely caused by the mongoose but also by high R. rattus populations on all six islands. In addition, we provide evidence that R. rattus has changed its activity time to become more nocturnal on mongoose-infested islands, possibly to avoid predation by the mongoose. As R. rattus became more nocturnal, the diurnal mongoose may have become the main predator on amphibians, reptiles, and poultry.

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